Recent advances in telecommunications and Digital Signal Processing (DSP), technologies, have created a growing demand for multimedia communication products for both business and home use. Multimedia communications often involve the simultaneous transmission of audio, video and data, such as graphics, fax or computer data, through an available shared communication link. To make efficient use of the available communication link, a number of techniques are required. For example, compression algorithms for compressing various media types are needed to reduce the bandwidth needed to transmit them. In addition, an efficient and flexible multiplexing method is needed to provide an acceptable quality of service, i.e., low multiplexing overhead and queuing delay, for each media type.
In traditional circuit-switched networks, different signals are multiplexed together using time division multiplexing (TDM). In TDM, a fixed bandwidth is typically allocated to each media for the duration of a call, and there is little flexibility to take advantage of the bursty nature of data, video and audio information.
To gain more flexibility and efficiency, packet multiplexing techniques have been proposed. Packet multiplexing has been widely adopted in ATM networks. In packet multiplexing, each information bitstream is segmented into packets, and packets from different bitstreams are multiplexed and sequentially transmitted over a communication channel. Each packet typically contains a header field and a payload field. A packet header contains a packet identifier which is used for recovering each individual information bitstream from a multiplexed packet sequence. The payload field of a packet may optionally contain some media-specific adaptation information in addition to actual information bits. Packets may be of fixed-length or of variable-length. Fixed-length packets used in ATM have the following advantages: fast segmentation and reassembly, no need for delineation flags, and easy synchronization. However, fixed-size packets are not suitable for use on low-speed links, because of efficiency and delay considerations. Variable-length packets simplify the implementation of the adaptation layers and allow a flexible design to trade-off delay against efficiency. Thus, variable-length packets are more appealing for use on low-speed links such as voiceband modem links.
When variable-length packets are used, flags are inserted between packets for delineation and synchronization. A widely used variable-length packet format is the HDLC-based framing structure, where HDLC represents High-level Data Link Control. In the HDLC format, the packet delineation flag, called the HDLC flag, is a one-byte binary word: "01111110". To avoid duplication of the HDLC flag in the information bitstream, HDLC bit stuffing is applied to the content of a packet between two flags by inserting a "0" bit after every five contiguous "1" bits. Overhead caused by HDLC bit-stuffing of a random bitstream is approximately 1.6%, but can be as high as 20% in the worst case. Another use of the HDLC flag is in bit-rate adaptation. When the aggregate bit rate is less than the channel rate, the HDLC flag can be repetitively sent during channel idle periods.
The effectiveness of a packet multiplexing scheme typically depends on its efficiency and delay. Efficiency is obtained by reducing packet overhead and by maximizing the bandwidth utilization. Increasing the packet size reduces the effective packet overhead, but increases the queuing delay. Thus, there is a need for a device and method that can achieve a good trade-off between efficiency and delay in a multimedia communication syste m